1. Field of the Invention
This invention applies to cases for protecting objects which are being publicly displayed, such as those used in museums or archival storage or shipping of sensitive artifacts; and more specifically applies to hermetically-sealed, air-tight cases for such applications.
2. Background of the Invention
Display cases are substantially transparent enclosures, usually made of plate glass or acrylic plastic in which objects are displayed for viewing. It is well known that many airborne pollutants, both particulate and chemical, find their way into display cases to contaminate object on display. Typical pollutants are dust, organic and carbon products from automobile exhausts, tires and brakes. Also, "smog" chemicals, include sulfur dioxide, nitrous oxide, carbon monoxide and hydrogen sulfide from catalytic converters. Each patron in an eight-hour day in the area will slough off approximately 1.5 grams of human skin cells, plus clothing fiber particles; and 3.5 cups of saliva containing water vapor and digestion enzymes; and often acetic acid, alcohol vapor mold spores, etc. from recently consumed food or drinks. Added to that list are chlorine and ammonium products used in normal cleaning processes in the building. Chemical and photochemical reactions then combine elements to produce hydrochloric acid, nitric acid and sulfuric acid. Most commercial display cases, even those housing extremely valuable objects that may be damaged by pollutants, are poorly sealed or not sealed at all. Display cases housing very high-priced merchandise in retail stores usually have glass or wood doors with no attempt to seal them at all, and as a result it is necessary to periodically dust, clean and rotate merchandise, with the attendant labor costs. When items on display are too soiled, faded or damaged by the ambient environment entering the cases, it is normal practice for stores to discount and even discard them at substantial annual cost.
Priceless objects in museums are also normally displayed in unsealed or poorly sealed cases. Typical museum cases, known as "vitrines" are comprised of a wooden base on which a five-sided glass or acrylic plastic box rests. As atmospheric pollutants seep into the cases and particulates show up in and on the displays as internal "dust" which is simply the visible accumulation of various chemical and organic pollutants. Then the cases must be opened and thoroughly cleaned. Invisible chemical pollutants in the case cause artifacts to be degraded until they are eventually rotated out of displays.
Unfortunately, the root cause of much internal case pollution it is not well known. The combination of lighting and air conditioning in buildings will cause diurnal-to-nocturnal temperature changes inside display cases. Each time an imperfectly-sealed case is heated by lighting or any other thermal source, the air within a display case expands and exhales through cracks and pores. Each time the unending weather cycles cause the natural barometric pressure to drop a little, the captured higher-pressure air within the case further exhales. Each time patrons view objects on display they radiate heat (approximately 300 watts per person) and breathe out warm air. The temperature increase also causes the case to exhale. Then, as any or all of these forces is reversed; the lights are turned off, the room temperature is lowered by air conditioning, the barometric pressure rises or warm bodies leave the room, the differential pressure inside the case drops to a pressure lower than the ambient external pressure. Then the case literally tries to inhale and suck any particulate or gaseous contaminants in the ambient air into the case. A typical display case slowly exhales during the day and sucks in the surrounding ambient air during the night. Since room cleaning is done after business hours, the vapors from cleaning solvents and particulates from sweeping, dusting and vacuuming are launched into the air just before the normal nocturnally-cooled inhaling mode begins. As a result ingestion of airborne contaminants, the present practice is to open up prior-art display cases periodically, remove all the artifacts, and then clean the interior of the cases and the artifacts. This is extremely expensive, as highly skilled personnel must remove fragile artifacts to accomplish the cleaning. It is also potentially damaging to the artifacts, as ancient materials are often very fragile and may be damaged by the cleaning processes or, if they are made of textiles, paper or other organic materials, even from being moved. Similar, problems exist for cases used for archival storage of pollution-sensitive objects in both museum and in industrial applications.